A study on the nature and role of normal, i.e. physiological platelet-asociated IgG (PAIgG) is proposed. The function of complement component C3 will also be sutdied. The major objectives are 1) characterization of the reactivity of PAIgG and determination of its specificity, 2) investigation of changes in membrane structure and function due to binding of PAIgG and C3 to platelets, 3) study of the significance of PAIgG and C3 for platelet function, and 4) correlation of PAIgG with platelet senescence. These studies constitute a continuation of research done during the past 2 years which has led to the development of a sensitive and simple test for quantification of this protein on platelets and in the serum. Most of the experiments will be carried out with human platelets, although a few studies are planned with rabbit platelets. The hypothesis on which this project is based postulates that PAIgG is a unique species of IgG probably with carbohydrate reactivity, which has opsonizing function, accumulates on platelets in the course of againg and aids in the recognition of senescent platelets by macrophages. PAIgG will be isolated by cell column chromatography using platelets immobilized on Sepharose to specifically remove IgG with binding affinity for platelets. The effects of PAIgG, isolated by cell column chromatography, and of C3 on membrane fluidity, membrane-related enzyme activities, transport functions and membrane protein distribution will be investigated. The latter will be analyzed by the use of fluorescent energy transfer between appropriate pairs of acceptor-donor fluorophores attached to different Ig or C3 molecules. The opsonizing capacity of PAIgG and C3 will be evaluated by in vitro assays of phagocytosis using monocytes or macrophages and will be related to the level of membrane-bound PAIgG, the number of available binding sites and the age of platelets. The effect of plasma on these parameters will be carefully analyzed. The amount of PAIgG will also be related to the survival of platelets. We expect that these studies will elucidate nature and function of IgG and C3 associated with normal platelets and thereby contributing to a better understanding of the important problem of clearance of senescent cells.